Dot matrix line printers include a plurality of dot printing mechanisms, each including a dot forming element. The dot printing mechanisms are mounted on a carriage such that the dot forming elements are located along a horizontal print line that lies perpendicular to the direction of paper movement through the printer. Located on the side of the paper remote from the dot forming elements is a platen and located between the dot forming elements and paper is a ribbon. During printing, the dot forming elements are selectively actuated to create one or more dots along the horizontal print line as the carriage is shuttled back and forth. The paper is incremented forwardly after each horizontal dot row is printed. A series of dot rows creates a horizontal row of characters, or a graphical image.
In general, dot matrix line printers fall into two categories. In the first category are dot matrix line printers wherein only the dot forming elements are shuttled. In the second category are dot matrix line printers wherein the entire print mechanism, e.g., the actuating mechanism as well as the dot forming elements, is shuttled. The present invention is useful with both categories of dot matrix line printers. More specifically, while the invention was developed for use in a dot matrix line printer wherein the entire print mechanism is shuttled, the invention also can be utilized in dot matrix line printers wherein only the dot forming elements are shuttled.
In the past, in many dot matrix line printers, the portion of the dot forming mechanism is to be shuttled has been mounted on, or forms part of, a carriage that is supported at either end by a suitable support mechanism, such as a linear bearing or a flexure. A flexure is an elongate piece of flat spring steel having one end attached to the frame of the printer and the other end attached to the carriage.
As described in U.S. patent application Ser. No. 844,092, entitled "Shuttle Drive For Flexure Mounted Carriages," by Lev Lipkovker et al., in the past, various types of carriage shuttling mechanisms have been used to shuttle the carriages of dot matrix line printers, including stepping motor, constant speed motor and linear motor driven mechanisms. Each such shuttling mechanism has various advantages and disadvantages.
The shuttling mechanism described in U.S. patent application Ser. No. 844,092, referenced above, is a weight unbalanced shuttling mechanism that comprises motor rotated unbalancing weights attached to a flexure supported carriage. The rotation of the unbalancing weights produces a vibration that causes reciprocating carriage movement. The mass and shape of the unbalancing weights is chosen to create a vibration that produces a carriage displacement. The rotary position (or phasing) of the unbalancing weights, one with respect to the other, is chosen to produce the desired force/displacement amplitude along the longitudinal axis of the carriage.
While the weight unbalanced shuttle drive system described in U.S. patent application Ser. No. 844,092 has a number of advantages over previously proposed drive mechanisms for shuttling the carriage of a dot matrix line printer, it also has disadvantages. One such disadvantage is the difficulty of maintaining a stable carriage displacement versus time relationship in the presence of external forces, such as an external bump being applied to the printer. If carriage displacement versus time changes when a printer is bumped, the carriage undergoes undesirable motions that result in the misplacing of printed dots. Misplacing of printed dots results in imperfectly formed characters, which is unacceptable in modern printers. While this problem can be avoided by sensing undesirable carriage motion and delaying printing until the effect of such motion ends, printing delay is an unacceptable resolution of this problem in modern high speed printing systems.
The present invention is directed to providing a twin counterweight shuttle drive for a reciprocably mounted carriage that avoids the foregoing disadvantage. More specifically, the invention is directed to providing a twin counterweight shuttle drive for a reciprocably mounted carriage, specifically a reciprocably mounted line printer carriage, that remains stable in the event of external bumps.
Another problem associated with the use of a weight unbalanced shuttle drive system of the type described in U.S. patent application Ser. No. 844,092 with a flexure supported carriage relates to the desirability that such systems have a low natural frequency. While a low natural frequency is desirable from energy consumption and force coupling (to the printed hammer) points of view, low natural frequency systems have a carriage displacement problem when printers are positioned on inclined surfaces. Further, each time the print mode of a line printer having a flexure mounted carriage is activated, the carriage is accelerated from zero through the system resonance point to the operating frequency of the printer. If the natural frequency of the system is low and carriage motion is uncontrolled, such acceleration causes a momentary carriage displacement much greater than the carriage displacement that occurs at the operating frequency. The time required for this undesired high amplitude excursion to be damped out defines how quickly printing can be commenced after the print mode of the printer is activated. In a system with a low damping ratio (e.g., 0.1), the typical damping time will fall in the range between 6 and 10 seconds. Obviously, a delay of 6 to 10 seconds between the time that a print command is given and when printing commences is totally unacceptable. If the damping ratio is raised to a level (approximately 0.5) that reduces the settling time to an acceptable level (e.g., 1 to 2 seconds), the force coupled by the flexures to the printer frame increases by a factor of 3 to 4 times, causing the printer to shake. Again, this is totally unacceptable.
The present invention is also directed to providing a twin counterweight shuttle drive for a reciprocably mounted dot matrix line printer carriage that is shock stabilized in a manner that avoids the foregoing disadvantages. More specifically, the invention is directed to eliminating shuttle amplitude changes when accelerating and maintaining the center point of the carriage peak-to-peak travel in the same relative position regardless of whether the printer is sitting level or inclined at some slight angle to the horizontal.